Chemistry Paper2 Ans

PEPERIKSAAN PERCUBAAN SPM 2008 NEGERI KEDAH DARUL AMAN MARKING SCHEME CHEMISTRY PAPER 2 Section A 1.

(a)

An acid that dissociates/ ionises completely in water to form a high concentration of hydrogen ions pipette From pink to colourless (i) HCl + NaOH  NaCl + H2O (ii) 0.1 x 20 = 1 Mb x 25 1

(b) (c) (d)

(e)

1 1 1 1 1 1

25 Mb = 2 Mb = 0.08 mol dm-3 10 cm3 // half the volume of hydrochloric acid Sulphuric acid is a diprotic acid whereas hydrochloric acid is a monoprotic acid. So, the sulphuric acid used has twice the number of hydrogen ions compared to hydrochloric acid .

(i) (ii)

Total

2.

(a)

(i)

(b)

(ii) (i)

1 1 1 1

10

Zinc hydrochloric acid / suphuric acid Zn + 2HCl  ZnCl2 + H2

1 1 1 1 1 1

Empirical formula is MO2

( c)

(d)

(ii) MO2 + 2H2  M + 2H2O The air in the combustion tube must be displaced before lighting the hydrogen gas// The heating, cooling and weighing is repeated until a constant mass is obtained

1 1

No. Magnesium is more reactive than hydrogen.

1 1 Total

1

10

3

(a)

Propene Vinyl chloride // Chloroethene

1 1

(b)

Advantage: Light // Cheap // convenient

1

(c)

Disadvantage: Non-biodegradable // produces toxic gas when burnt i Increase hardness // reduce corrosion / rusting // improve the appearance//to make it more attractive

1 1

ii

To make Medals // swords // statues // bells

1

iii

Copper

1

Tin iv

Atoms/particles in copper are arranged in an orderly manner in layers. The presence of tin atoms disturbs the orderly arrangement of copper atoms. This will make it difficult for the layers of tin atoms to slide on each other. Total

4

(a)

1 1 1 10 1

X

(b)

(c)

i

Isotopes are atoms (of the same element) with the same number of protons/proton number but different number of neutrons/nucleon number.

1

ii

Any example of isotope

1

i

Sample answer; carbon-14,cobalt-60, sodium-24 etc 12

1

ii

23

1

11 Y

(d)

i

Liquid

1

2

(ii)

iii

1

Becomes faster/ more active

1

iv

correct curve boiling point

o

Temperature / C

1 1

65

Time / min Total 5

10

(a)

1

The stopper is drawn in the correct position(half in, half out) refer the diagram (b) (c) (d)

2HCl + CaCO3  CaCl2 + H2O + CO2 32 cm3 50 s = 0.64 cm3s-1 (i) Both axes are labelled with the correct units scales are suitable(cover ½ graph paper) All points are transferred correctly Smooth curve

1 1

(ii)

1 1 1

1 1 1 1

A tangent is drawn on the graph Calculation 0.246 cm3s-1 + 0.024 // range [0.222 - 0.270] Total

3

10

6

(a) (b)

(c) (d) (e)

Hydrogenation// addition (of hydrogen) (i) Phosphoric acid // H3PO4

1 1

(ii) (iii) (i) (ii)

C2H4 + H2O  C2H5OH Fermentation Ethanoic acid Acidified potassium dichromate(VI) solution // Acidified potassium manganate(VII) solution C2H5OH  C2H4 + H2O (i) Esterification

1 1 1 1

(ii)

1

- has a sweet smell// fruity smell - a neutral compound - colourless liquid - slightly soluble in water - readily soluble in organic compounds [ Choose any one ]

(iii)

1 1

1 H

O

H

H

H–C–C–O –C–C–H H

H

H Total

4

10

Section B 7

(a) (b)

Hydrogen(gas) 2H+(aq) + 2e  H2(g) Properties 1. Type of cell 2. Energy change 3. Electrodes

4. Ions in electrolyte 5. Half equation

6. Observation

1 1 Cell A Voltaic cell Chemical  electrical Positive terminal: Copper Negative terminal: Magnesium Cu2+, SO42-, H+ and OHions Positive terminal: Cu2+ + 2e  Cu Negative terminal Mg  Mg2+ + 2e Positive terminal: Copper plate becomes thicker Magnesium becomes thinner/dissolve

(c)

(i) (ii)

Cell B Electrolytic cell Electrical  chemical Anode: Copper Cathode: Copper

1 1 1

Cu2+, SO42-, H+ and OHions Anode: Cu  Cu2+ + 2e Cathode: Cu2+ + 2e  Cu Anode: Copper dissolves//become thinner Cathode: Copper becomes thicker

Improve the appearance//to make it more attractive To prevent/ reduce corrosion/ rusting Procedure: 1. Iron ring is then connected to the negative plate on the battery while the silver plate is connected to the positive terminal of the battery//Iron ring is made as cathode while silver plate is made as anode 2. Both plates are immersed into the silver nitrate solution. 3. The circuit is completed

Functional apparatus set-up Label correctly: silver plate Silver nitrate solution Iron ring Cathode: Ag+ + e  Ag Observation: Grey /silvery solid is deposited Anode : Ag  Ag+ + e Observation: Anode/silver become thinner//dissolve

1 1 1 1 1

8

1 1

2

1

1 1

1 1 1 1 1 1 Total

5

2

max 8/9 20

8

(a)

(b)

(i)

Atom R is located in Group 17, Period 3

(ii)

Electron arrangement of atom R is 2.8.7. It is located in Group 17 because it has seven valence electron. It is in Period 3 because it has three shells filled with electron Atoms P and R form covalent bond. To achieve the stable electron arrangement, atom P needs 4 electrons while atom R needs one electron. Thus, atom P shares 4 pairs of electrons with 4 atoms of R, forming a molecule with the formula PR4 // diagram

(i)

1 1 1 1 1

5

1 1 1 1 1

R

R

P

R

R

(ii)

Atom Q and atom R form ionic bond. Atom Q has the electron arrangement 2.8.1. and atom R has the electron arrangement 2.8.7. To achieve a stable (octet )electron arrangement, atom Q donates 1 electron to form a positive ion// equation Q Q+ + e

1

Atom R receives an electron to form ion R-//equation and achieve a stable octet electron arrangement. R+e R-

1 1

Ion Q+ and ion R- are pulled together by the strong electrostatic forces to form a compound with the formula QR// diagram

1

1 1

-

+ Q

R

11

6

(c)

The ionic compound/ (b)(ii) dissolves in water while the covalent compound / (b)(i)does not dissolve in water. Water is a polar solvent that can cause the ionic compound to dissociate into ions. Covalent compounds are non-polar and can only dissolve in organic solvents.

1 1 1 1

4

OR The melting point of the ionic compound/ (b)(ii) is higher than that of the covalent compound/ (b)(i) . This is because in ionic compounds ions are held by strong electrostatic forces. High energy is needed to overcome these forces. In covalent compounds, molecules are held by weak intermolecular forces. Only a little energy is required to overcome the attractive forces.

1 1 1 1 1

max 4/5

OR The ionic compound/(b)(ii) conducts electricity in the molten or aqueous state whereas the covalent compound/(b)(i) does not conduct electricity. This is because in the molten or aqueous state, ionic compounds consist of freely moving ions. Covalent compounds are made up of molecules only

1 1 1 1 Total

9

(a)

(i)

Based on the equation, iron(II) ion is oxidised to iron(III) ion So, iron(II) ion acts as a reducing agent Based on the equation, iron(II) ion is reduced to iron So, iron(II) ion acts as an oxidising agent

(ii)

(b)

1 1 1 1

Mg → Mg 2 + + 2e

1

Oxidation number of magnesium increases from 0 to +2, So magnesium undergoes oxidation

1

Cu

2+

+ 2e → Cu

oxidation number of copper decreases from +2 to 0, so copper(II) ion undergoes reduction (c)

4 20

(i)

Reduction is a reaction that involves gain of electron. Oxidation is a reaction that involves loss of electron.

7

4

1 1

4

1 1

2

( ii)

At the negative terminal: Iron(II) ion releases one / loses one electron and is oxidised to iron(III) ion. Fe2+  Fe3+ + e The green coloured solution of iron(II) sulphate turns brown. The electron flows from the negative terminal// carbon immersed in iron(II) sulphate solution to the positive terminal// carbon immersed in bromine water.

1 1 1 1 1

At the positive terminal: Bromine accepts electron and is reduced to bromide ions, BrBr2 + 2e  2BrThe brown coloured bromine water turns colourless. The deflection of the galvanometer needle shows that there is a flow of current

1 1 1 1 1

10

Total 20 10

(a)

(b)

Exothermic reaction is a reaction that releases heat to the surrounding The energy content of the products is lower than the energy content of the reactants Endothermic reaction is a reaction that absorbs heat from the surrounding The energy content of the products is higher than the energy content of the reactants

1

1

4

A reacts with B to form C and D A and B are the reactants while C and D are the products Heat energy is released //The reaction is exothermic Total energy content of A and B/reactants is higher than total energy content of C and D/products

1 1 1 1

4

(c) thermometer Copper container water Spirit lamp + ethanol

8

1 1

250 cm3 of water is measured using a measuring cylinder and is then poured into a copper container. The copper container is placed on a tripod stand. The initial temperature of the water is measured and recorded. A spirit lamp containing ethanol is weighed and recorded. The spirit lamp is placed directly below the copper container(as in the diagram.) The wick of the spirit lamp is lighted. The water is stirred using a thermometer. When the increase in water temperature is 30 oC, the flame is put off and the highest temperature reached is recorded. The spirit lamp is weighed again. Results: Initial temperature of water = t1 oC Highest temperature of water = t2 oC Rise in temperature = (t2 - t1)oC = T oC Mass of lamp + ethanol before combustion = m1 g Mass of lamp + ethanol after combustion = m2 g Mass of ethanol burnt = (m2 – m1) g

1 1 1 1 1 1 1 1 1

1

1

Number of mol of ethanol = (m2 – m1) = n 46

1

Heat change = 250 x 4.2 x T = qJ

1

Heat of combustion = -

q n x 1000

kJ mol-1

1 Max 12/14 Total

9

20